This invention relates to the processing of cast titanium articles to improve the microstructure of such articles.
The major use of titanium castings is in the aerospace, chemical and energy industries. The aerospace applications generally require high performance cast parts, while the chemical and energy industries primarily use large castings where corrosion resistance is a major consideration in design and material choice.
The casting of titanium and titanium alloys presents a special problem due to the high reactivity of the material in the molten state. This requires special melting, mold-making practices, and equipment to prevent alloy contamination. At the same time, titanium castings present certain advantages when compared to castings of other metals. The microstructure of as-cast titanium is desirable for many mechanical properties. It has good creep resistance, fatigue crack growth resistance, fracture resistance, and tensile strength. Titanium alloy castings also readily lend themselves to full densification by hot isostatic pressing (HIP) because they dissolve their own oxides at high temperatures allowing a complete closure of all nonsurface-connected porosity by diffusion bonding. However, on the debit side, some cast-parts mechanical properties, particularly those which are initiation-related, such as smooth fatigue, are currently inferior to those exhibited by ingot metallurgy (IM) parts.
The melting practice used for cast-part production is essentially the same as for alloy ingot melting. Accordingly, it is possible to cast all titanium alloys produced by ingot metallurgy. The major difference between ingot metallurgy and cast metallurgy parts stems from the subsequent hot working and heat treatment of ingots or their products, which allow microstructural manipulations not possible in the cast part, such as, for example, equiaxed recrystallized alpha.
Accordingly, it is an object of the present invention to provide a process for improving the microstructure of a cast titanium article.
Other objects, aspects and advantages of the present invention will become apparent to those skilled in the art after reading the detailed disclosure of the invention as well as the appended claims.